1. Field of the Invention
This invention relates to an apparatus and method for detecting and locating quench zones in a superconducting coil using a sensor positioned adjacent to the superconducting coil and being in heat transfer communication with the coil, and evaluating echo pulses reflected due to changes in the resistance on the transmission line.
2. Background Information and Description of the Prior Art
Superconducting inductors (or magnets) are capable of storing large amounts of energy. In addition, these superconducting inductors (or magnets) also generate high magnetic fields. Superconducting inductors are very efficient for these purposes because no energy is lost due to resistive heating in the superconducting current path. Superconductors are operated at cryogenic temperatures. Such temperatures are extremely low, being slightly above absolute zero. For a given ambient magnetic field, a superconducting material has a critical temperature above which the material is no longer superconducting. Above the critical temperature, resistance increases in the area where the material is no longer superconducting.
If a region of a superconducting coil becomes resistive and no longer acts as a superconductor, this is known as becoming "normal" or "quenching". When a region of the superconductor becomes normal, joule heating of the normal region takes place. If sufficient joule heating occurs, the normal zone propagates and grows larger. This can be a catastrophic condition leading to severe damage in the coil as temperatures rapidly increase in the normal region. For this reason, early detection of a normal zone is extremely important. Early detection of the normal zone can permit an opportunity to dissipate the energy stored in the coil often by shunting the energy away from the normal region or sometimes by quenching the entire coil. Catastrophic damage due to overheating can thereby be avoided.
Several methods for detecting normal regions in a superconducting coil have been known. The primary technique has involved the use of voltage taps. In accordance with this method, voltages are measured at various points along the coil with the objective of correlating changes in voltage with the existence of normal regions. A drawback of using voltage taps in the case of a superconducting inductor is that in addition to the resistive voltage associated with a normal zone, a superconducting inductor exhibits inductive voltages resulting from the charging or discharging of energy in the coil. Although the normal zones must be detected when they are small, the inductive voltage between the two taps is usually much larger than the resistive voltage even for a severe quench. Therefore, the large inductive voltage must be eliminated from the total voltage measured at the taps. Typically, this involves subtracting out the inductive voltage by comparing the signal to a reference value in order to determine the resistive voltage. The reference voltage is obtained from a sense coil monitoring the magnetic field, or it may be a voltage obtained from a voltage tap at a different location on the inductor. As noted, the technique involves subtracting two voltage measurements to determine the small resistive component. This can result in inaccurate readings due to the complications inherent with such a technique.
Furthermore, the information obtained by using the voltage tap technique is limited to determining whether a normal zone exists between the two voltage taps. The exact position of the normal zone between the taps cannot be determined with this technique. In addition, the inductive voltages in the coil may sum up to tens of kilovolts across the inductor terminal. In practice, this places severe constraints on the electronic components which are used for the voltage taps. In order to accurately locate the position of a quench along a superconducting coil, in addition to merely detecting that a quench exists, numerous voltage taps must be used. However, this results in numerous penetrations into the dewar leading to heat leaks, which can raise the temperature of the adjacent portion of the coil and in turn contribute to quench zones occurring. Even small heat leaks affect the cost of operation and thermal efficiency of the overall system since many times the energy of a leak is required to reduce it.
It has been known to provide two superconducting strands to create a sensor which is then cowound with the superconducting coil. At one end of the sensor, the two strands may be shorted together. The overall sensor tends to be noninductive since one leg of the sensor has a positive inductive voltage and the second leg has a negative inductive voltage. Thus, the problem of an inductive voltage being detected is not presented in this technique. A voltage limited current source provides current to the sensor and a voltmeter detects resistive voltage in the sensor. While the main superconducting inductor itself is superconducting the cowound electrical sensor is also superconducting. Normal zones in the inductor drive the corresponding section of the cowound sensor normal through heat conduction. The resistance in the sensor can then be measured to detect the existence of a normal zone. As with the voltage tap methods, the cowound sensor method is limited to detecting the existence of a normal zone. The position of the normal zone along the sensor cannot be determined using this method. It is often important to know the location of a quench for purposes of maintenance and repair. The superconducting magnet may be located underground or in another location which is not readily accessible. If abnormal conditions or quench zones continually occur in a particular area of the coil, this area can be immediately targeted for repair or maintenance. Otherwise, the entire line would have to be inspected. Additionally, one of the primary concerns in superconducting technology is to avoid heat leaks into the refrigerated zone. However, unnecessary penetration into the dewar which may be necessary to locate a repair zone can lead to unnecessary heat leaks into the dewar.
Other limitations in the technique include a lack of selectivity between a long, slightly heated quench and a short hot quench as the total resistance measured in both such situations can be equal.
There remains a need, therefore, for a quench measuring device and method which will allow early detection of normal zones in a superconductor but which will also provide information about the location of the normal zone along the coil. There remains a further need for a device which will not require excessive penetrations into the dewar which can lead to heat leaks and increase the risk of voltage breakdowns. In addition, there remains a need for a device which will provide information as to the magnitude of the quench.
It is an object of this invention to provide a quench measuring system which detects a quench zone and provides information as to the location of the quench zone along the superconducting coil.
It is a more particular object of the invention to provide such a system which minimizes the possibility of heat leaks into the system and which eliminates the need for multiple components and voltage taps to be built into the system.
It is a further object of the invention to provide a system in which a differentiation can be made between a long slightly heated quench and a short hot quench.